I love art challenges - and Cath & Roc's eight week postcard challenge on Mail Art World http://www.mailartworld.com/ is one of the best. Members are sent a collage page of arbitrary - and weird - images to use. Then at the end of the week we race to swap.
Here's mine. The gargoyle looks mournfully at cakes and chocs... and his Ma and Pa. He is weeping.